The present invention relates to a process for preparing zirconia-alumina base composite powders and, more particularly, to a hydrothermal process for preparing zirconia-alumina base composite ultra-fine powders of high purity.
Zirconia-alumina base composite ceramics including an alumina matrix with zirconia dispersed therein are referred to as zirconia toughened alumina, which have for their object the improvement of the toughness of alumina. Zirconia toughened alumina finds use in cutting tools and engineering material applications due to the fact that their toughness is higher than that of conventional alumina ceramics.
Conversely, alumina-dispersed zirconia, in which alumina is dispersed throughout a matrix of partially stabilized zirconia or tetragonal zirconia, is used to reduce the weight, increase the hardness and minimize the cost of high-toughness zirconia ceramics.
Conventionally, zirconia-alumina base composite powders have been prepared by mechanical mixing using a ball mill, etc. However, it has been found that considerable difficulty is involved in homogeneous mixing of fine powders, which are of the submicron order and which excel in sinterability, by such mechanical means.
There has also been proposed a process wherein mixed precipitates are obtained from a solution containing zirconium and aluminum components by neutralization/co-precipitation, calcination and pulverization. It has been noted, however, that the composition of the mixed precipitates fluctuates, partly because the concentration of ammonia used as the precipitant varies locally, and partly because the precipitation conditions vary at the initial and final stages. For these reasons, delicate differences in the operation conditions have led to large differences in the properties of the precipitant.
In recent years, techniques using hydrothermal oxidation, gas-phase reaction and plasma have also been developed for preparing highly homogeneous zirconia-alumina base composite fine powders; however, these techniques are characterized by high costs and low productivity.